Damped ski and method of making



ANowl, 1970 D. B. CALDWELL DAMPED sx1 'AND METHOD 0F MAKING Filed Nov.

l U.S. Cl. 280-1L13 ABSTRACT OF THE DISCLOSURE Ski flutter is greatly reduced and maneuverability enhanced by sandwiching a viscoelastic layer (formed, eg., from certain acrylic copolymers) between the upper surface of the ski and a stretch-resistant constraining layer (e.g., aluminum sheet).

BACKGROUND OF THE INVENTION This invention relates to skis and to methods of improving the performance thereof.

Prior to the last three decades, virtually all skis were made of wood. Relatively few woods, however, have the strength, wear resistance, ability to retain camber and flexibility needed to make good skis. Even with suitable woods, great care must be exercised in selection, since any defect will alter the performance characteristics and may actually render the ski dangerous to use. Further, no matter what wood is chosen, it is diicult to form two skis having identical physical properties. It is not surprising, then, that the mass production of matched wood skis of high quality has never been possible.

In the last quarter century a great deal of effort has been devoted to the manufacture of skis having a shell or skeleton of metal and/or reinforced plastics, the interior core typically being filled with wood blocks, foamed plastic or other material which provides compressive strength. Such skis are uniform, predictable, and have signiiicantly greater wear resistance and useful life. On the other hand, such skis have a somewhat different feel than wooden skis, typically displaying a greater degree of flutter, or vibration, particularly when used on steep or icy slopes, or when making sharp turns. Although it has been felt that utter was a minor consequence, attempts have been made to damp skis by modifying their internal construction, seeking to make their performance more closely resemble that of wooden skis. Prior to the present invention, however, such attempts have been at best only modestly successful.

SUMMARY The present invention provide-s an improved ski construction in which mass-produced skis can be made to have the feel and lack the flutter characteristic of wooden skis. The resultant product is one which combines the best physical attributes of both the earlier wooden Skis and the modern carefully constructed synthetic products. If desired, skis can be initially manufactured in accordance with the invention, but it is almost equally satisfactory to modify a pair of commercial skis to incorporate the features` of the invention, very little time or effort being required. A surprising and unexpected feature of the modified ski construction is a noticeably enhanced maneuverability, as is particularly manifested by increased speed in slalom competition.

United States Patent O Patented Nov. 3, 1970 lCC Skis made in accordance with the present invention have as their uppermost surface a stretch-resistant constraining layer such as aluminum or stainless steel sheet,

glass filament-reinforced plastic, etc. Interposed between the lower surface of the constraining layer and the underlying rigid constructional face of the ski is a viscoelastic layer which has a thickness of at least 0.015 inch (about 0.4 mm.), a glass transition temperature in the range of -55 C. to 85 C., and, at temperatures throughout the range of 25 C. to -{-5 C. and frequencies of 10-20 Hz., a loss tangent value of at least 0.5 (preferably at least 0.7) and a dynamic shear modulus in the range of 0.7-700 kg./cm.2. The constraining layer and the viscoelastic layer are further selected so that the dynamic stretching stiffness of the former, measured in force per unit width, is significantly higher than (preferably at least 10 times as great as), the dynamic shear stiffness of the latter, measured in force per unit volume in the foregoing temperature and frequency ranges, all measurements being in c.g.s. units.

A commercial ski is conveniently modified with the aid of a strip of damping material formed by applying a viscoelastic layer to one face of a suitable constraining layer (e.g., thin aluminum sheet) and then adhering the viscoelastic layer to the upper surface of the ski. The entire upper surface of the ski may be provided with the strip of damping material, although satisfactory results are obtained by applying the damping material in a strip extending longitudinally from the bindings to the front touchdown point of the ski. For mens skis which are at least 78 inches (approximately 200 cm.) long, a 32-inch (approximately 81 cm.) long damping strip has been found extremely suitable. Preferably the viscoelastic layer has aggressive pressure-sensitive adhesive characteristics, facilitating attachment of the damping strip and permitting removal if and when desired. It is also contemplated, however, to employ a viscoelastic layer which lacks adequate adhesiveness, a separate pressure-sensitive or other adhesive being used to bond the viscoelastic layer to the upper surface.

Suitable viscoelastic materials may be based on polyisobutylene. Particularly preferred, however, are copolymers af alkyl acrylate and one or more copolymerizable acrylic monomers such as acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile, acrylamide and methacrylamide. The alkyl acrylate may be a single monomer having about -6-10 carbon atoms in its alkyl group which is not highly branched, i.e., more than half of the alkyl carbon atoms are in a straight chain terminating at the oxygen bridge. Where the alkyl acrylate is a mixture of monomers, their alkyl groups should have an average of about 6-10 carbons, and less than half of the alkyl groups should be highly branched. Because of their age-resistance, affinity for metals, adhesion characteristics and viscoelastic properties at the range of temperatures encountered, copolymers containing -97% isooctyl acrylate and correspondingly 10-3% acrylic acid are especially preferred.

Depending on the degree of damping which is sought and the personal preference of the user, the strip of damping materal can be the same width as the ski; alternatively, one or more strips of lesser width, preferably arranged symmetrically can be applied. Likewise, a plurality of damping strips can be superposed and adhered to the ski; in such case, the width of each strip may be the same or, if desired, upper strips may be narrower than lower strips. It is also apparent that for sophisticated skiers, the degree of damping may be tailored to the individual or the snow conditions, damping strips being added or removed as desired. Generally, however, it has been found that a ski which has been damped is more satisfactory to an individual for all skiing conditions.

lIn order to demonstrate the effectiveness of the present invention in reducing ski lflutter, a test has been devised to measure the duration of vibrations set up in a ski. In this test, the rear portion of a ski (i.e., that portion lying more than 31/2 inches (about 9 cm.) behind the mid point of the ski) is held in a massive rigid clamp. A light weight vibration pickup is then attached approximately 6 inches (l cm.) to the rear of the front touchdown point. The ski is then set into vibration by lightly plucking the tip, vibration thereafter continuing at the resonant frequency of the ski (which is almost always in the range of -15 HZ.) and gradually dying away. The Output of the vibration pickup is fed into a recorder having a strip chart moved beneath a marking pen at a speed of mm. per second. From the chart, the resonant frequency is calculated and the average height ratio of several succeeding peaks measured, thereby obtaining the average peak to peak decrement ratio. The formula for vibration decay rate, D, expressed in decibels per second, is D=8.68 K,1 where K is loge of the amplitude decrement ratio per second (i.e., loge of the product of frequency in Hz. and the average decrement ratio). A typical decay rate for a wooden ski is about 15 db/sec.; the vibration decay rates for conventional non-wooden skis are usually in the range of 2-5 db/sec., with a few approaching 15 db/sec. It has been found that, regardless of the nature of the ski to which damping strips made in accordance With this invention are applied, the decay rate can easily be increased by 10-20 decibels per second. -If desired, of course, the dimensions of the strip can be varied t0 achieve a greater 0r lesser degree of damping.

When a ski is modified in accordance with this invention by applying a strip of damping material to the upper surface, the strip is preferably chosen so that the weight increase is held to a minimum. In evaluating the effectiveness of a given damping strip, it has been found useful to employ the damping index, an arbitrarily selected parameter which is defined as the vibration decay rate increase in db/sec. (compared to a bare ski) divided by the weight of the damping strip in grams. Il is preferred that the weight increase does not exceed 200 grams and that the damping index be at least about 0.1,

It is also preferred that the thickness of the entire damping strip be held to as low a caliper as feasible, the overall thickness preferably being no more than about 0.1 inch (2.5 mm.), not only to minimize Weight but also to facilitate moving the ski sideways through snow. It has been found, however, that it is important for the overall thickness to be at least about 0.02 inch (0.5 mm.) if significant improvement is to be obtained. Satisfactory results have been achieved where the constraining layer is sheet aluminum having a thickness in the range of 0.015-0.050 inch (about 0.4-1.25 mm.) and the viscoelastic layer has a thickness of at least about 0.015 inch (about 0.4 mm.). Particularly good results are obtained with a 0.032-inch aluminum sheet (about 0.8 mm.) and a 0.025-inch (about 0.6 mm.) viscoelastic layer. If stainless steel is substituted for the aluminum, the constraining layer can be made thinner; the weight for comparable performance, however, is about the same.

BRIEF DESCRIPTION OF THE DRAWING Understanding of the invention will be facilitated by reference to the accompanying drawing, in which:

1 Cf., eg., Greiger and Hamme Laboratories Report No. U.S. Navy 1 (April 1961), entitled The Concept of Damping of Structureborne Sound and Vibration for Noise Control.

FIG. l is a side elevational view showing one form of ski made in accordance with the invention;

FIG. 2 is a top view of a conventional ski modied in accordance with the present invention; and

FIG. 3 is an enlarged cross-sectional view of a portion of the ski shown in FIG. 2, taken along section lines 3*3.

DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS FIG. l depicts a ski 10 having a body portion 11, the lower surface of which has front touchdown 12 and rear touchdown 13. Damping strip 14 comprises constraining number 15 and viscoelastic layer 16, the latter being firmly adhered to the upper surface of body 11. Numeral 17 designates the general location where a ski boot (not shown) is attached when ski 10 is used.

FIGS. 2 and 3 depict a ski 10a having body 11a, with front touchdown 12a and rear touchdown 13a. To the upper forward surface of body 11a are attached damping strips 14a, each individually similar to damping strip 14 as shown in FIG. l, comprising constraining member 15a and viscoelastic layer 16a. Damping strips 14a extend longitudinally from approximately the forward portion of boot location 17a to front touchdown 12a.

Understanding of the invention will be further enhanced by referring to the following illustrative but nonlimitative examples, in which all parts are by weight unless otherwise noted.

EXAMPLES A glass fiber-filled 95.5:4.5 isooctylacrylate: acrylic acid copolymer, for use as a viscoelastic material in practicing the invention was prepared in accordance with Example 1 of Kalleberg and Turner U.S. Pat. 3,062,683 and knife-coated on a glassine-kraft paper carrier web having a silicone-treated release surface on both sides. The coated carrier web was dried in an air-circulating oven at about C. for 2 minutes, 155 C. for 3 minutes and finally about C. for 6 minutes to provide a dried coating about 1 mil (25 microns) thick, whereupon the coated carrier web was Wound upon itself in roll form for storage. To insure that the coating was sufficiently dry, a small piece of the coating was weighed and then conditioned for two hours at 25 C. and 30% relative humidity and reweighed. No loss in weight indicated that the coating was substantially free from volatile material and thus could be used for a viscoelastic layer. Loss of weight during the conditioning period would have indicated the presence of trapped volatiles which could result in undesirable change in damping performance during use. The loss tangent and dynamic shear modulus, measured with apparatus such as that described by E. J. Fitzgerald in The Physical Review, vol. 108, pp. 69() et seq. (1957) and extrapolated by conventional methods, were found to be as set forth below:

1 The glass fibers have no appreciable effect on these values and can, l desired, be omitted; the fibers serve primarily to prevent any tendency ofthe viseoelastie material to ooze.

Dynamic shear stiffness is calculated for a given viscoelastic layer by dividing the dynamic shear modulus by the stiffness. Glass transition temperature, determined by refractive index measurements, was -71 C.

Damping strips were prepared as follows: A 30-inch diameter rnetal roll was wrapped with S-mil (0.127 mm.) biaxially-oriented and heat-set polyethylene terephthalate (polyester) film. The roll was then rotated while' the coated carrier web was pressed against the film by a spring-loaded From the foregoing table it is observed that a 0.016 (0.41 mm.) aluminum restraining layer is not quick enough to obtain optimum results. It is inferred that it stretches slightly, thereby diminishing the effect of the visoelastic material. The' 0.032 (0.81 mm.) aluminum layer, how- 5 ever, is eminently satisfactory and olers a clear cut weight savings over the 0.040 (1.02 mm).

It was also concluded that a 0.025" (0.64 mm.) layer of viscoelastic material represented a good compromise of l0 performance characteristics, thinner layers being more prone to shock loose `and thicker layers offering, at best, no more than nominal improvements.

The following table summarizes the effect of adding damping strips [32 (81 cm.) long, full ski width, 0.032 l5 (0.81 mm.) 7075T6 aluminum, 0.025" (1 mm.) 95.5:4.5

isooctyl acrylate: acrylic acid copolymer viscoelastic adhesive] to several type's of skis.

TABLE II Decay rates, db/sec.

with Bare damping Manufacturer andmodel ski strip Increase laminating troll to transfer the copolymer coating to the polyester lm, and the carrier web was stripped away. This was continued until the successive convolutions of the copolymer coating reached a total thickness of 25 mils (approximately 0.6 mrn.), the' carrier web being allowed to contact the outermost convolution of the coating. 'I'he temporary laminate of the liner, copolymer coating and polyester film were then slit parallel to the axis of the roll, removed therefrom, chilled to about 5 C. and the polyester film removed. The coated liner was then placed on a table, coated side up, se'veral Widths of 32-inch long 7075T6 aluminum strips, ranging from 0.016 to 0.040 inch (approximately 0.4 to 1 mm.) thick, placed on the coated surface, :and the damping material and liner trimmed to size with a razor blade. (The Youngs modulus for 7075T6 aluminum strips is approximately 7,000,000 kg./ cm2; stretching stiffness is calculated as the product of Ski construction 3,537,717 7 8 width are likewise effective. The skis tested were the same 7' down point, at least one elongated strip of a damping maas those listed in Table I, the constraint layer being 0.032- terial comprising in combination: inch (0.81 mm.) aluminum in all cases. a viscoelastic layer adhered over said upper surface of TABLE III Visco- Damping elastic Decay strip Tcmporlayer Decay rate Damping width, ature, thickness, rate, increase, weight, Damping om. D C. Ski 111111. db/see, (ib/seu. grants index Example:

0. G 21. 2 11. G 24. 5 14. .1 24. 2 14. 18. 2 8. G 16. 3 G. 7 17. i) 8. 3

8.8 25. 4 16. G 108. 7 0. 153 29. 0 20. 2 116.9 0. 173 25g. 20. 6 141 0. 1116 13.9 10. 8 54 0.200 14. 3 11. 2 58. 8 0. 190 14. 7 11. 6 70. 7 0.164 17. 8 14. 7 108. 7 0.135 17. 6 14. 5 116. J 0. 124 1U, 8 1G. 7 141 0.110 E). 2 6. 1 54 0. 113 10. 3 7. 2 58. 8 0. 122 13. 3 10. .Z 70. T U. 144 7. 7 2t). 2 12. 5 108. 5 0. 115 -23 ...do 0.64 2.1 14.1 118.0 0.121 -23 Golden Jet" 4.6 -23 do 0. 38 13. 6 El. l) 108. 5 l). 083 -23 do 0.64 16.3 11.7 118.6 0.0111! What I claim is:

l. The method of reducing the normal tendency of a ski to chatter and vibrate on rough or icy substrates, comprising applying to the upper surface of said ski, in an area lying between the bindings and a touchdown point, at least one strip of a damping material comprising in combination:

a stretch-resistant constraining layer bearing, on the face adjacent the ski surface,

a viscoelastic layer, said viscoelastic layer having:

a glass transition temperature in the range of -55 C. to 85 C.,

a thickness of at least about 0.4 mm., and at temperatures throughout the range of -25 C. to C. and at frequencies in the range of 10- Hz.,

a loss tangent of at least 0.5 and a dynamic shear modulus in the range of 0.7-700 kg./cm.2, the dynamic stretching stiffness of said constraining layer being significantly greater than the dynamic shear stiffness of said viscoelastic layer,

whereby the rate of vibration decay of said ski at its resonant frequency is significantly greater than its rate of vibration decay in the absence of said strip of damping material.

2. A ski having laminated to the upper surface thereof, in an area lying between the bindings and the front touchthe ski, said viscoelastic layer having a thickness of at least about 0.4 mm. and at temperatures throughout the range of 25 C. to -1-5" C. and frequencies of 10-20 Hz., a loss tangent of at least 0.5 and a dynamic shear modulus in the range of 0.7-700 kg./cm.2,

a constraining layer laminated to the upper surface of the viscoelastic layer, said constraining layer having a dynamic stretching stiffness at least l0 times as great as the dynamic shear stiffness of said viscoelastic layer, the overall thickness of said damping material being 0.5-2.5 mm.

3. The ski of claim 2 wherein the viscoelastic layer has a loss tangent of at least 0.7.

4. The ski of claim 2 wherein the constraining layer is sheet aluminum having a thickness of 0.4-1.25 mm.

References Cited UNITED STATES PATENTS 2,264,535 12/1941 Klemm. 2,995,379 8/1961 Head. 3,194,572 7/1965 Fischer. 3,300,226 1/1967 Reed. 3,475,035 10/1969 Nason.

LEO FRIAGLIA, Primary Examiner M. L. SMITH, Assistant Examiner 

